1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs). More specifically, the present invention relates large-pixel multi-domain vertical alignment LCDs, which can be manufactured with smooth substrates.
2. Discussion of Related Art
Liquid crystal displays (LCDs), which were first used for simple monochrome displays, such as calculators and digital watches, have become the dominant display technology. LCDs are used routinely in place of cathode ray tubes (CRTs) for both computer displays and television displays. Various drawbacks of LCDs have been overcome to improve the quality of LCDs. For example, active matrix displays, which have largely replaced passive matrix displays, reduce ghosting and improve resolution, color gradation, viewing angle, contrast ratios, and response time as compared to passive matrix displays.
However, the primary drawback of conventional twisted nematic LCDs is the viewing angle is very narrow and the contrast ratio is low. Even the viewing angle of active matrixes is much smaller than the viewing angle for CRT. Specifically, while a viewer directly in front of an LCD receives a high quality image, other viewers to the side of the LCD would not receive a high quality image. Multi-domain vertical alignment liquid crystal displays (MVA LCDs) were developed to improve the viewing angle and contrast ratio of LCDs. FIGS. 1(a)-1(c) illustrate the basic functionality of a pixel of a vertical alignment LCD 100. For clarity, the LCD of FIG. 1 uses only a single domain. Furthermore, for clarity, the LCDs of FIGS. 1(a)-1(c) (and FIG. 2) described in terms of gray scale operation. Furthermore, FIGS. 1(a)-1(c) is simplified to clarity and omits many processing layers. For example, between substrate 110 and electrode 120, actual displays would likely include various metal layers used for electrical connections as well as insulating layers that separate the metal layers.
LCD 100 has a first polarizer 105, a first substrate 110, a first electrode 120, a first alignment layer 125, liquid crystals 130, a second alignment layer 140, a second electrode 145, a second substrate 150, and a second polarizer 155. Generally, first substrate 110 and second substrate 150 are made of a transparent glass. First electrode 120 and second electrode 145 are made of a transparent conductive material such as ITO (Indium Tin Oxide). First alignment layer 125 and second alignment layer 140, which are typically made of a polyimide (PI) layer, align liquid crystals 130 vertically in a resting state. In operation, a light source (not shown) sends light from beneath first polarizer 105, which is attached to first substrate 110. First polarizer 105 is generally polarized in a first direction and second polarizer 155, which is attached to second substrate 150, is polarized perpendicularly to first polarizer 105. Thus, light from the light source would not pass through both first polarizer 105 and second polarizer 155 unless the light polarization were to be rotated by 90 degrees between first polarizer 105 and second polarizer 155. For clarity, very few liquid crystals are shown. In actual displays, liquid crystals are rod like molecules, which are approximately 5 angstroms in diameter and 20-25 angstroms in length. Thus, there are over 12 million liquid crystal molecules in a pixel that is 120 μm width by 300 μm length by 3 μm height. Although not shown, many liquid crystal displays (particularly active matrix LCDs) include a passivation layer on bottom of first electrode 120. The passivation layer serves as an insulating layer between the first electrode 120 and devices and conductors that may be formed on the substrate. The passivation layer is commonly formed using silicon nitrides.
In FIG. 1(a), liquid crystals 130 are vertically aligned. In the vertical alignment, liquid crystals 130 would not rotate light polarization from the light source. Thus, light from the light source would not pass through LCD 100 and gives a completely optical black state and a very high contrast ratio for all color and all cell gap. Consequently MVA LCDs provide a big improvement on the contrast ratio over the conventional low contrast twisted nematic LCDs. However, as illustrated in FIG. 1(b), when an electric field is applied between first electrode 120 and second electrode 145, liquid crystals 130 reorientate to a tilted position. Liquid crystals in the tilted position rotate the polarization of the polarized light coming through first polarizer 105 by ninety degrees so that the light can then pass through second polarizer 155. The amount of tilting, which controls the amount of light passing through the LCD (i.e., brightness of the pixel), is proportional to the strength of the electric field. Generally, a single thin-film-transistor (TFT) is used for each pixel. However for color displays, a separate TFT is used for each color component (typically, Red, Green, and Blue)
However, the light passing through LCD 120 is not uniform to viewers at different viewing angles. As illustrated in FIG. 1(c), a viewer 172 that is left of center would see a bright pixel because the broad (light rotating) side of liquid crystals 130 face viewer 172. A viewer 174 that is centered on the pixel would see a gray pixel because the broad side of liquid crystals 130 is only partially facing viewer 174. A viewer 176 that is right of center would see a dark pixel because the broad side of liquid crystals 130 is barely facing viewer 176.
Multi-domain vertical alignment liquid crystal displays (MVA LCDs) were developed to improve the viewing angle problems of single-domain vertical alignment LCDs. FIG. 2 illustrates a pixel of a multi-domain vertical alignment liquid crystal display (MVA LCD) 200. MVA LCD 200 includes a first polarizer 205, a first substrate 210, a first electrode 220, a first alignment layer 225, liquid crystals 235, liquid crystals 237, protrusions 260s, a second alignment layer 240, a second electrode 245, a second substrate 250, and a second polarizer 255. Liquid crystals 235 form the first domain of the pixel and liquid crystals 237 form the second domain of the pixel. When an electric field is applied between first electrode 220 and second electrode 245, protrusions 260 cause liquid crystals 235 to tilt in a different direction than liquid crystals 237. Thus, a viewer 272 that is left of center would see the left domain (liquid crystals 235) as black and the right domain (liquid crystals 237) as white. A viewer 274 that is centered would see both domains as gray. A viewer 276 that is right of center would see the left domain as white and the right domain as black. However, because the individual pixels are small, all three viewers would perceive the pixel as being gray. As explained above, the amount of tilting of the liquid crystals is controlled by the strength of the electric field between electrodes 220 and 245. The level of grayness perceived by the viewer directly related to the amount of tilting of the liquid crystals. MVA LCDs can also be extended to use four domains so that the LC orientation in a pixel is divided into 4 major domains to provide wide symmetrical viewing angles both vertically and horizontally.
Thus, multi-domain vertical alignment liquid crystal displays, provide wide symmetrical viewing angles, however, the cost of manufacturing MVA LCDs are very high due to the difficulty of adding protrusions to the top and bottom substrates and the difficulty of properly aligning the protrusions on the top and bottom substrates. Specifically, a protrusion on the bottom substrate must be located at the center of two protrusions on the top substrate; any misalignment between the top and bottom substrates will reduce the product yield. Other techniques of using physical features to the substrates, such as ITO slits, which have been used in place of or in combination with the protrusions, are also very expensive to manufacture. Furthermore, the protrusions and ITO slits inhibit light transmission and thus reduce the brightness of the MVA LCDs.
However, MVA LCDs have been developed that do not require the use of physical features (such as protrusions or ITO slits) on the substrate. Specifically, these MVA LCDs use fringe fields to create multiple-domains. Without the requirement of physical features the difficulty of aligning the physical features of the top and bottom substrate is eliminated. Thus, MVA LCDs using fringe fields have higher yield and are less expensive to manufacture than MVA LCDs that use physical features on the substrates.
FIGS. 3(a) and 3(b) illustrate the basic concept used to create a multi-domain vertical alignment liquid crystal display (MVA LCD) 300 without resorting to physical features on the substrates. Specifically FIG. 3 shows pixels 310, 320, and 330 in between a first substrate 305 and a second substrate 355. A first polarizer 302 is attached to first substrate 305 and a second polarizer 357 is attached to second substrate 355. Pixel 310 includes a first electrode 311, liquid crystals 312, liquid crystals 313 and a second electrode 315. Pixel 320 includes a first electrode 321, liquid crystals 322, liquid crystals 323 and a second electrode 325. Similarly, pixel 330 includes a first electrode 331, liquid crystals 332, liquid crystals 333 and a second electrode 335. Although not shown, many liquid crystal displays include a passivation layer on top of electrodes 311, 321, and 331. The electrodes are typically constructed using a transparent conductive material such as ITO. Furthermore, a first alignment layer 307 covers the electrodes on first substrate 305. Similarly a second alignment layer 352 covers the electrodes on second substrate 355. Both LC alignment layers 307 and 352 provide a vertical LC alignment. As explained in more detail below, electrodes 315, 325, and 335 are held at a common voltage V_Com. Therefore, to ease manufacturing, electrodes 315, 325, and 335 are created as a single structure (as shown in FIGS. 3(a) and 3(b)). MVA LCD 300 operates pixels 310, 320, and 330 using alternating polarities. For example, if the polarities of pixels 310 and 330 are positive then the polarity of pixel 320 would be negative. Conversely, if the polarities of pixel 310 and 330 are negative then the polarity of pixel 320 would be positive. Generally, the polarity of each pixel would switch between frames, but the pattern of alternating polarities is maintained in each frame. In FIG. 3(a), pixels 310, 320, and 330 are in the “OFF” state, i.e. with the electric field between the first and second electrodes turned off. In the “OFF” state some residual electric field may be present between the first and second electrode. However, the residual electric field is generally too small to tilt the liquid crystals.
In FIG. 3(b), pixels 310, 320, and 330 are in the “ON” state. 3(b) uses “+” and “−” to denote the voltage polarity of the electrodes. Thus, electrodes 311, and 331 have positive voltage polarity and electrodes 321 has negative voltage polarity. Substrate 355 and electrodes 315, 325, and 335 are kept at common voltage V_com. The voltage polarity is defined with respect to the V_com voltage, where a positive polarity is obtained for voltages higher than V_com, and a negative polarity is obtained for voltage smaller than V_com. Electric field 327 (illustrated using field lines) between electrodes 321 and 325 causes liquid crystals 322 and liquid crystals 323 to tilt. In general, without protrusions or other features the tilting direction of the liquid crystals is not fixed for liquid crystals with a vertical LC alignment layers at 307 and 352. However, the fringe field at the edges of the pixel can influence the tilting direction of the liquid crystals. For example, electric field 327 between electrode 321 and electrode 325 is vertical around the center of pixel 320 but is tilted to the left in the left part of the pixel, and tiled to the right in the right part of the pixel. Thus, the fringe field between electrode 321 and electrode 325 cause liquid crystals 323 to tilt to the right to form one domain and cause liquid crystals 322 to tilt to the left to from a second domain. Thus, pixel 320 is a multi-domain pixel with a wide symmetrical viewing angle
Similarly, the electric field (not shown) between electrode 311 and electrode 315 would have fringe fields that cause liquid crystals 313 to reorientate and tilt to the right in the right side in pixel 312 and cause liquid crystals 312 to tilt to the left in the left side in pixel 310. Similarly, the electric field (not shown) between electrode 331 and electrode 335 would have fringe fields that cause liquid crystals 333 to tilt to the right in the right side in pixel 330 and cause liquid crystals 332 to tilt to the left in the left side in pixel 330.
Alternating polarity of adjacent pixels amplifies the fringe field effect in each pixel. Therefore, by repeating the alternating polarity pattern between rows of pixels (or columns of pixels), a multi domain vertical alignment LCD is achieved without physical features. Furthermore, an alternating polarity checkerboard pattern can be used to create four domains in each pixel.
However, fringe field effects are relatively small and weak, in general. Consequently, as pixels become larger, the fringe fields at the edge of the pixels would not reach all the liquid crystals within a pixel. Thus, in large pixels the direction of tilting for the liquid crystals not near the edge of the pixels would exhibit random behavior and would not produce a multi-domain pixel. Generally, fringe field effects of pixels would not be effective to control liquid crystal tilt when the pixels become larger than 40-60 μm. Therefore, for large pixel LCDs pixel division methods are used to achieve multi-domain pixels. Specifically, for color LCDs, pixels are divided into color components. Each color component is controlled by a separate switching element, such as a thin-film transistor (TFT). Generally, the color components are red, green, and blue. The color components of a pixel are further divided into color dots.
The polarity of each pixel switches between each successive frame of video to prevent image quality degradation, which may result from twisting the liquid crystals in the same direction in every frame. However, the dot polarity pattern switching may cause other image quality issues such as flicker if all the switching elements are of the same polarity. To minimize flicker, the switching elements (e.g. are transistors) are arranged in a switching element driving scheme that include positive and negative polarities. Furthermore, to minimize cross talk the positive and negative polarities of the switching elements should be arranged in a uniform pattern, which provides a more uniform power distribution. The three main switching element driving schemes are switching element point inversion driving scheme, switching element row inversion driving scheme, and switching element column inversion driving scheme. In the switching element point inversion driving scheme, the switching elements form a checkerboard pattern of alternating polarities. In the switching element row inversion driving scheme, the switching elements on each row have the same polarity; however, each switching element in one row has the opposite polarity as compared to the polarity of switching elements in adjacent rows. In the switching element column inversion driving scheme, the switching elements on each column have the same polarity; however, a switching element in one column has the opposite polarity as compared to the polarity of switching elements in adjacent columns. While the switching element point inversion driving scheme provides the most uniform power distribution, the complexity and additional costs of switching element point inversion driving scheme over switching element row inversion driving scheme or switching element column inversion driving scheme may not be cost effective. Thus, most LCD displays for low cost or low voltage applications are manufactured using switching element row inversion driving scheme while switching element point inversion driving scheme is usually reserved for high performance applications.
Pixels may include various key components arranged to achieve high quality low cost display units. For example, pixel can include color components, color dots, fringe field amplifying regions (FFAR), switching elements, device component areas, and associated dots. Displays using these various components are described in U.S. Patent Application “Cite various KYO Patents KYO-001 KYO-003, KYO-005, KYO-006”, which are incorporated herein by reference.
Device component area encompasses the area occupied by the switching elements and/or storage capacitor as well as the area that was used to manufacture the switching elements and/or storage capacitors. For clarity, a different device component area is defined for each switching element.
Associated dots and fringe field amplifying regions are polarized areas that are not part of the color components. Associated dots cover the device component areas. Generally, the associated dots are manufactured by depositing an insulating layer over the switching element and/or storage capacitors. Followed by depositing an electrically conductive layer to form the associated dot. The associated dots are electrically connected to specific switching element and or other polarized components (such as color dots). The storage capacitors are electrically connected to specific switching element and color dot electrodes to compensate and offset the capacitance change on the liquid crystal cells during the switching-on and switching-off processes of the liquid crystal cells. Consequently, the storage capacitors are used to reduce the cross-talk effects during the switching-on and switching-off processes of the liquid crystal cells. A patterning mask is used when it is necessary to form the patterned electrode for the associated dots. A color layer is added to form a light shield for the associated dot. In general, the color layer is black however some displays use different color to achieve a desired color pattern or shading. Generally, the color layer is achieved by depositing a color filter layer on the corresponding ITO glass substrate. Specifically, a patterned color filter layer is deposited between second substrate 150 and second electrode 140 with pattern corresponding to the color for the color dot and associated dots. However, some displays may also place a patterned color filter layer underneath the electrode layer of the color dots, associated dots, or DCA on the substrate.
In some displays, the associated dot is an area independent of the switching elements. Furthermore, displays have additional associated dots not directly related to the switching elements. Generally, the associated dot includes an active electrode layer such as ITO or other conductive layer, and is connected to a nearby color dot or powered in some other manner. For opaque associated dots, a black matrix layer can be added on the bottom of the conductive layer to form the opaque area. The black matrix can be fabricated on the ITO glass substrate side to simplify the fabrication process. The additional associated dots improve the effective use of display area to improve the aperture ratio and to form the multiple liquid crystal domains within the color dots. Some displays also use associate dots to improve color performance. For example, careful placement of associated dots can allow the color of nearby color dots to be modified from the usual color pattern.
Fringe field amplifying regions are more versatile than associated dots. Specifically, fringe field amplifying regions may have non-rectangular shapes, although generally, the overall shape of the fringe field amplifying regions can be divided into a set of rectangular shapes. Furthermore, fringe field amplifying regions extend along more than one side of a color dot. In addition, fringe field amplifying regions may be used in place of associated dots in some displays. Specifically, in these displays the fringe field amplifying region cover the device component areas but also extend along more than one side of color dots adjacent to the device component areas.
In general, the color dots, device component areas, and associated dots are arranged in a grid pattern and are separated from adjacent neighbors by a horizontal dot spacing HDS and a vertical dot spacing VDS. When fringe field amplifying regions are used in place of associated dots, part of the fringe field amplifying regions would also fit in the grid pattern. In some displays multiple vertical dot spacings and multiple horizontal dot spacings may be used. Each color dot, associated dot, and device component area has two adjacent neighbors (e.g. color dots, associated dots, or device component areas) in a first dimension (e.g. vertical) and two adjacent neighbors in a second dimension (e.g. horizontal). Furthermore, two adjacent neighbors can be aligned or shifted. Each color dot has a color dot height CDH and a color dot width CDW. Similarly, each associated dot has an associated dot height ADH and an associated dot width ADW. Furthermore, each device component area has device component area height DCAH and a device component area width DCAW. In some displays, color dots, associated dots and device component areas are the same size. However in many displays color dots, associated dots and device component areas could be of different size or shapes. For example in many displays associated dots have a smaller height than color dots.
When a LCD panel is subject to external touch pressure on the panel substrate, touch mura occurs. Touch mura effects due to physical disturbance of the liquid crystals is a major issue with vertically aligned liquid crystal displays (both single domain and multiple domain). Touch mura effects refer to irregular patterns or regions causing uneven screen uniformity. Physical disturbance of the liquid crystals may be caused by shaking, vibration, and pressure on the display. In particular, vertically aligned liquid crystal displays are very susceptible to touch mura effects caused by pressure on the display. Specifically, pressure on a vertically aligned liquid crystal display may flatten the liquid crystals and cause a disturbance effect on the display. Unfortunately, devices incorporating touch screen functionality (i.e. users of a device apply pressure on the surface of the display as a means of providing user input to the device) has become increasingly popular which may hinder acceptance of vertically aligned liquid crystal displays. Hence there is a need for a method or system to minimize touch mura effects in a vertically aligned liquid crystal display.